The invention relates to an anti-scatter grid for use in an X-ray device in order to reduce scattered radiation produced in an object to be examined, which grid includes a plurality of absorber laminations for the absorption of the scattered radiation and a channel medium which is transparent to X-rays and is arranged between the absorber laminations. The invention also relates to a collimator for a single photon emitter which includes a plurality of laminations in order to form collimator channels and also a channel medium provided between the laminations.
When an object to be examined is irradiated by means of X-rays, not only primary radiation is produced in the object to be examined, for example, a patient, but also scattered radiation so that a xe2x80x9cscatter fogxe2x80x9d is superposed on the X-ray image. Because of this additional exposure, the contrast of the X-ray image is reduced to an extent which is dependent on the scattered radiation intensity and the signal-to-noise ratio of the detail to be imaged is also degraded.
In order to reduce the scattered radiation, therefore, X-ray devices are provided with an anti-scatter grid which is arranged between the object to be examined and the X-ray detector and which transmits the primary radiation emanating from the focal spot of the X-ray tube but substantially absorbs the scattered radiation from the object to be examined which is incident on the absorber laminations at various angles.
An X-ray device of this kind is known from U.S. Pat. No. 1,164,987. The absorber laminations are usually made of lead which has a small volume in combination with a high absorptivity. The channel medium in the intermediate spaces between the absorber laminations is paper, fiber or aluminum.
It is an object of the invention to provide an anti-scatter grid in which the scattered radiation is suppressed as much as possible whereas the primary radiation is transmitted as much as possible. Moreover, the manufacture of the anti-scatter grid should be as simple as possible, geometrically accurate and economical. This object is achieved in accordance with the invention by means of an anti-scatter grid of the kind set forth which is characterized in that the channel medium is a non-elastic high-resistance foam.
The invention is based on the recognition of the fact that air would be the ideal channel medium, but also that the absorber laminations must be arranged in a mechanically rigid and positionally accurate manner so as to achieve an as high as possible homogeneity. It has now been found that a non-elastic high-resistance foam is ideal from these points of view. Such a foam has a high transmissivity for the primary radiation because the density is approximately from 15 to 30 times smaller than that of paper. Furthermore, a high mechanical precision can be achieved for the arrangement of the absorber laminations, because the shape of the material is stable and the material can be suitably worked. Moreover, a desired large number of lines, that is, a large number of absorber laminations per unit of length (for example, cm), of the scatter grid can also be achieved. Furthermore, production defects can be repaired during the manufacture, because the absorber laminations and the channel medium are not glued together. Moreover, the channel material is quite inexpensive.
Preferably, a polymethacrylimide high-resistance foam is used as the channel medium, for example, a foam as marketed under the name Rohacel(copyright) This material can be simply worked on fast wood or plastic processing machines, for example, by splitting, cutting, grinding or milling. The use of lubricants is permissible. The final shape and the final dimensions can be imparted to the channel elements after the working (after which they are in parallel), for example, by cold pressing. Thermal deformation and bonding or resinification so as to obtain the ultimate shape are also possible. This material thus enables the manufacture of an anti-scatter grid which has substantially the same properties as an anti-scatter grid utilizing air as the channel medium.
The channel medium in a further preferred embodiment comprises individual, preformed channel elements, a channel element being arranged each time between two absorber laminations. The channel elements are thus preformed and subsequently combined with the absorber laminations so as to form the anti-scatter grid, said assembly being realized either by means of an adhesive or, as in a further preferred embodiment, by holding the assembly together by means of a frame.
Alternatively, it is also possible in principle to form individual slits in a large piece of high-resistance foam, for example, by means of a saw, a hot water jet, a laser or a hot wire, and to arrange the absorber laminations subsequently in said slits. The slits may extend in parallel but also conically.
The anti-scatter grid may be constructed as a flat anti-scatter grid in which the absorber laminations are arranged in parallel. However, the anti-scatter grid is preferably constructed as a focused anti-scatter grid in which the absorber laminations are aligned each time along a line which extends through the focal point of the X-ray source. To this end, said preformed channel elements are preferably shaped so as to be conical with a suitable inclination. It may also be arranged that the overall anti-scatter grid is not flat but curved and hence is shaped as a spherical cap, thus enabling even better focusing with respect to the focal point of the X-ray source and hence an even higher homogeneity.
The individual channel elements in a further preferred embodiment are configured in the form of a grid. This means that parts are removed, preferably by sawing (parallel or conically), from the individual, comparatively flat, elongate channel elements, so that the channel element forms a kind of grid or network when viewed from the side (perpendicularly to the direction of the primary radiation). An even better absorption of the scattered radiation can thus be achieved.
The invention also relates to an X-ray device for forming X-ray images of an object to be examined, which device includes an X-ray source, an X-ray detector and an anti-scatter grid which is arranged between the object to be examined and the X-ray detector as described above. An X-ray device of this kind may be a conventional X-ray system, for example, for projection imaging or a C-arm X-ray system, but also a computed tomography system.
The invention also relates to a collimator, notably for a gamma camera of a single photon emitter (SPECT) or for a positron emitter (PET). A non-elastic high-resistance foam can again be advantageously used (for example, in order to save weight) as a channel medium in a collimator of this kind which comprises a plurality of laminations for forming collimator channels and also a channel medium between the laminations. A collimator of this kind is intended notably to transmit only a part of the gamma quanta emanating from the object to be examined to the further elements of a gamma camera which is situated behind the collimator. Only quanta which are incident practically perpendicularly to the surface of the collimator which faces the object to be examined can pass the collimator, whereas quanta which are incident at an angle are absorbed in the collimator walls. In order to achieve an as accurate as possible arrangement of the laminations and to minimize the absorption for the quanta to be transmitted, the described non-elastic high-resistance foam can be used for the channel medium, notably also a polymethacrylimide high-resistance foam such as that marketed under the name Rohacelg. The collimators may then have a linear or grid-like configuration in parallel or focused form. Moreover, the laminations may also be crossed and focused.
Finally, the invention also relates to a gamma camera, a single photon emitter for forming images of a radiating object by means of a gamma camera, the gamma camera being provided with a collimator of the described kind in order to define the projection direction of the image, as well as to a positron emitter.
The following description, claims and accompanying drawings set forth certain illustrative embodiments applying various principles of the present invention. It is to be appreciated that different embodiments applying principles of the invention may take form in various components, steps and arrangements of components and steps. These described embodiments being indicative of but a few of the various ways in which some or all of the principles of the invention may be employed in a method or apparatus. The drawings are only for the purpose of illustrating an embodiment of an apparatus and method applying principles of the present invention and are not to be construed as limiting the present invention.